1. Field of the Invention
The present invention relates to coatable urea-aldehyde binder precursor compositions having low free aldehyde content which are catalyzed to cured binders by a cocatalyst. The catalyst is described as a "cocatalyst" because it has two components: an ammonium salt (such as ammonium chloride, ammonium nitrate, ammonium thiocyanate, and the like) and a Lewis acid (such as aluminum chloride, ferric chloride, and the like). The cocatalyst is especially useful in the production of coated abrasive articles.
2. Discussion of Related Art
Generally, coated abrasives comprise a backing onto which a plurality of abrasive particles are bonded thereto. In one major form of the coated abrasive, the abrasive particles are secured to the backing by means of a first binder coat, called a make coating, which is adhered to the backing. Abrasive particles are applied while the make coating is in its uncured state, followed by precure of the make coating. Over the make coating abrasive particles is applied a second binder coat, commonly called a size coating. The purpose of the size coating is to reinforce the abrasive particles.
In a second major form of coated abrasive, the abrasive particles are dispersed in a binder to form an abrasive composite, and this abrasive composite is bonded to the backing by means of the binder. Coated abrasives are used in a variety of different applications from gate removal on forged metal parts to finishing eye glasses. Additionally, coated abrasives are converted into a wide variety of different forms including endless belts, sheets, cones, discs, and the like.
Examples of popular coated abrasive backings include paper, nonwoven webs, cloth, vulcanized fiber, polymeric film and treated versions thereof, and combinations thereof. The polymeric film, especially polyester film, has found wide commercial success in fine grade abrasives, i.e., the fine particle size of the abrasive particles. These fine grades are those in which the average particle size of the abrasive particle is less than about 150 micrometers, typically less than 100 micrometers. Polymeric film is consistently very flat and smooth, thus it does not have surface roughness like the other fibrous backings. This flatness and smoothness results in most, if not all, of the abrasive particles being substantially in one plane, and thus substantially all of the abrasive particles are able to contact the workpiece being abraded and typically a higher cut rate results. If the coated abrasive backing is rough, especially in the fine grades, the tendency is that not all of the abrasive particles will be in one plane and thus not contacting the workpiece at the same time. This in turn leads to a coarser surface finish and a lower rate of cut.
However, one problem associated with the smooth polymeric films is the adhesion of binders to that type of surface, in particular urea-aldehyde binders. Primers are routinely utilized on the front surface of the polymeric film to increase the adhesion of the make coating. Examples of primers include mechanical and chemical primers. It is one object of the present invention to provide improved coated abrasives of the type having a polymeric film backing having coated thereon an abrasive coating comprising a urea-aldehyde binder.
The use of acid/base reactions to control the addition and condensation reactions of urea-formaldehyde (UF) dates back to the 1918 work of Hanns John. It is generally accepted that a nucleophilic component is necessary for an amino-carbonyl condensation via reactions 1-3 (all aqueous): ##STR1##
Although the addition reaction (reaction 2) is both acid and base catalyzed, the condensation reaction (reaction 4) is exclusively acid catalyzed: ##STR2##
The nucleophilic component necessary for amino-carbonyl condensations can be provided by any of a variety of proton donors. The most common classes are mineral acids, OH--acidic compounds, acidic SH, NH and CH moieties, and some olefins.
UF was first patented for use as an adhesive for coated abrasives by Minnesota Mining and Manufacturing Company ("3M") in the mid 1930's (Great Britain Pat. No. 419,812). Since that time a number of different coated abrasive products have been made with acid catalyzed UF resins. Today, the two most common catalysts used with UF resins are aluminum chloride (AlCl.sub.3) and ammonium chloride (NH.sub.4 Cl).
Although urea-aldehyde resins have enjoyed great success in coated abrasives, the need to reduce the use of solvents and unreacted reactants which contribute to release of volatile organic hydrocarbons (VOC) in the process of making coated abrasives, and the need to increase the quality of the abrasives while maintaining or increasing their level of performance is challenging the industry.
Meanwhile, the appearance to the user of the coated abrasive is important. It has been interestingly found that, when attempting to increase the abrading performance of coated abrasives made using urea-aldehyde resins when aluminum chloride is used alone as the catalyst, a higher temperature than normal must be used to cure the urea-aldehyde resin, which in turn leads to curling of edges of paper-backed coated abrasives. (The use of aluminum chloride as a catalyst for urea-formaldehyde resins in the making of coated abrasive articles is known.) Therefore, it would be advantageous if the abrading performance of paper-backed coated abrasives made using urea-aldehyde resins could be increased without sacrificing the appearance or increasing the waste of coated abrasive.
When the AlCl.sub.3 catalyst is used alone, the gel time, pot life and peak exotherm temperatures are all dependent on the concentration of the AlCl.sub.3. Thus, the performance of the coated abrasive is dependent upon the concentration of the AlCl.sub.3, and the cure conditions (time and temperature).
In order to achieve a good performing product using factory cure conditions (i.e temperature ranging from about 65.degree. C. to about 95.degree. C.), the concentration of AlCl.sub.3 should be near 1 weight percent, based on weight of binder precursor. The drawback with a 1 weight percent concentration of AlCl.sub.3 is that the pot-life may be too short for batch operations typically used in the factory with urea-aldehyde resins having low (about 0.1 to about 1.0 weight percent) free aldehyde content, based on total weight of aldehyde.
When NH.sub.4 Cl is used alone as the catalyst, the gel time, pot life and peak exotherm temperatures are all independent of the NH.sub.4 Cl concentration, affording an advantage over the use of a Lewis acid catalyst. However, the activity (ability of the catalyst to catalyze the reaction) of the NH.sub.4 Cl was dependent on the free formaldehyde concentration in the binder precursor composition due to the following reaction: EQU 6CH.sub.2 (OH).sub.2 +4NH.sub.4 Cl.fwdarw.(CH.sub.2).sub.6 N.sub.4 +4HCl+12H.sub.2 O 5)
With low free aldehyde resins, such as that known under the trade designation "AL3029R", from Borden Chemical, the NH.sub.4 Cl does not activate the condensation reaction (4) very readily until the temperature of the reaction is increased above that normally used. However, as mentioned above, increased temperature tends to curl the edges of paper-backed coated abrasives and without performance improvements. The performance of the coated abrasive is independent of the NH.sub.4 Cl concentration. Thus, the drawbacks of this system are the long gel times, and only moderate performance levels are obtained with typical factory cure conditions.
Therefore, it would be an advance in the art to provide a binder precursor composition (preferably a solution or dispersion) which includes a urea-aldehyde resin and cocatalyst system and coated abrasives which meet these needs. It is the primary object of the present invention to provide such compositions which will, when cured, provide a coated abrasive binder having uniformity of physical properties as is previously known, but which also allow higher production runs of coated abrasives without curling of the edges of the coated abrasive web and increased abrasion performance.